Interpretive Summary: Reducing dietary protein in trout feeds will reduce production costs if growth performance can be maintained. A study was conducted to determine if balancing plant-based diets on an available amino acid basis would reduce total dietary protein needs. The diets were formulated to contain either 45 or 35% intact crude protein. The diets were balanced for essential amino acids. The diets were fed to 20g rainbow trout for a period of 12-weeks. Amino acid supplementation improved weight gain. Feed conversion ratio was poorest for the unsupplemented low protein diet compared to all other treatments. Protein retained as growth was improved. Intraperitoneal fat ratio decreased with amino acid supplementation at both crude protein levels. Muscle ratio increased by 10% when amino acids were supplemented to the 45CP diet and by an average of 13.6% when amino acids were supplemented to the 35CP diet. In conclusion, dietary crude protein content of plant-based diets for rainbow trout can be reduced from 46% to 41.5% by supplementing lysine, methionine, and threonine with no reduction in growth and an improvement in protein retention efficiency and muscle ratio.

Technical Abstract:
Reducing dietary protein in trout feeds will reduce production costs if growth performance can be maintained. A study was conducted to determine if balancing plant-based diets on an available amino acid basis would result in a reduction in total protein level. The diets were formulated to contain either 45 or 35% intact crude protein. To these basal diets, lysine, methionine, threonine and glycine were supplemented to be equivalent to 450g/kg protein from rainbow trout muscle on an available amino acid basis. A fifth diet was formulated to contain 35% crude protein from plant proteins and supplemented as for the 35AA diet but only with lysine, methionine and threonine. The diets were fed to 20g rainbow trout for a period of 12-weeks. Amino acid supplementation improved weight gain. Feed conversion ratio was poorest for the 35CP diet compared to all other treatments. Protein retention efficiencies were improved in the 35AA and 35AA-Gly dietary treatments compared to the 45CP treatment. Intraperitoneal fat ratio decreased with amino acid supplementation at both crude protein levels. Muscle ratio increased by 10% when amino acids were supplemented to the 45CP diet and by an average of 13.6% when amino acids were supplemented to the 35CP diet. In conclusion, dietary crude protein content of plant-based diets for rainbow trout can be reduced from 46% to 41.5% by supplementing lysine, methionine, and threonine with no reduction in growth and an improvement in protein retention efficiency and muscle ratio.